In past several years, increasing data amount has accompanied a problem of increasing costs for storage. Considering the above, a concept of cold storage has been attractive to reduce the costs. Cold storage is an inexpensive storage device at the sacrifice of its performance and is exemplified by a tape storage device and an optical disk storage device, which each can be regarded as a library unit.
Such cold storage is used in conjunction with hot storage in a storage system. Hot storage is high in both performance and price and is exemplified by a Solid State Drive (SSD) and a Hard Disk Drive (HDD). In such a storage system, data less frequently accessed as time passes is migrated from the hot storage to the cold storage.
An SSD and a HDD each have a configuration that a medium that stores data is integrated with the drive that makes writing and reading accesses to the medium. This configuration makes it possible to start, immediately upon receipt of a request to read and write data from and into the medium, data writing into and data reading from the medium.
In contrast to the above, when writing and reading data into and from a medium, tape storage and optical disk storage insert a medium to be accessed into a drive. For example, tape storage uses a magnetic tape as a medium and optical disk uses an optical disk as a medium.
This means that tape storage and optical disk storage each include a medium and a drive which are isolated from each other and the cost of such type of storage can be reduced by reducing the number of drives. In such tape storage and optical disk storage, media are stored in the shelf of the storage casing and a few drives are also placed in the shelf. When the storage is making an access to a medium, a magazine carrier (robot) brings the medium to be accessed to the drive. It takes a time in units of minutes to change a medium to be inserted into drive, depending on the scale of the storage system, of course.
Accordingly, cold storage such as tape storage and optical disk storage preferably stores a series of data successively accessed in the same medium where possible, so that the time taken to access the data in the medium is minimized.
When each half of a series of data that takes two minutes to be read is dividedly stored in one of medium #0 and the medium #1, it takes the following time to read the entire series of data from the media #0 and #1. Assuming that it takes two minutes to change a medium placed in a drive, it takes two minutes to insert the medium #0 into the drive; one minute to read data from medium #0; two minutes to insert the medium #1 into the drive; and one minute to read data from medium #1. Consequently, it takes six minutes in total to obtain the series of data. In contrast, in cases where the same amount of data is stored in a single medium #2, the time taken to obtain the series of data is reduced to four minutes including two minutes to insert the medium #2 into the drive and two minutes to read data from the medium #2.    [Patent Literature 1] Japanese Laid-open Patent Publication No. 2004-334969    [Patent Literature 2] Japanese Laid-open Patent Publication No. 2005-322099    [Patent Literature 3] Japanese Laid-open Patent Publication No. SHO 60-49431    [Patent Literature 4] Japanese Laid-open Patent Publication No. HEI 10-187505